It is widely known that a speech is synthesized by performing filtering, which indicates a vocal tract characteristic, with respect to a sound source signal indicating a vibration of a vocal cord. Further, quality of a synthesized speech is improved and may be used inappropriately. Thus, it is considered that it is possible to prevent or control inappropriate use by inserting watermark information into a synthesized speech.
However, when an audio watermarking is embedded into a synthesized speech, there is a case where sound quality is deteriorated.